The present invention relates to a corona discharging device for an electrophotographic copier, laser printer or similar image forming equipment of the type using an electrophotographic procedure.
Image recording equipment of the type described has a corona discharging device for charging the surface of a photoconductive element which serves as an image carrier. The corona discharging device is implemented by a corona discharger such as a corotron or a scorotron. The corona discharger produces a corona discharge in response to a high voltage which is generated by a high-tension power supply. The output current of the power supply is controlled to remain constant so that the current for charging the photoconductive element may remain constant, whereby the charge potential on the element is stabilized against changes in ambient conditions such as temperature, humidity, and atmospheric pressure. Specifically, a prior art corona discharging device has a corona discharger which is made up of a corona wire and a shield electrode which surrounds the corona wire, that side of the shield electrode which faces a photoconductive element being open. A high-tension power supply applies a high voltage to the corona wire. The output current of the power supply is divided into a charging current for charging the photoconductive element and a shield electrode current which flows through the shield electrode. This brings about a drawback that when the division ratio, or distribution ratio, of the output current of the power supply is changed due to the contamination of the shield electrode, the charging current and therefore the charge potential on the photoconductive element changes despite that the output current is constant. Another drawback with the prior art device is that an air gap has to be provided between the corona wire and the shield electrode, obstructing the miniaturization of the corona discharger. Moreover, since the high voltage is fed to the corona wire even in an unusual condition of the equipment such as when one has forgotten to mount the photoconductive element, there is a fear of an electrical shock and an arc discharge between the corona wire and the shield electrode.